A polymer electrolyte membrane (PEM) fuel cell (or proton exchange membrane fuel cell) includes a polymer electrolyte membrane that separates an anode compartment, where oxidation of a fuel occurs, and a cathode compartment, where reduction of an oxidizer occurs. The anode and cathode are essentially constituted by a porous support, such as a porous carbon support, on which particles of a noble metal (e.g., platinum) are deposited. The PEM typically provides a conduction medium for protons from the anode to the cathode as well as providing a barrier between the fuel and the oxidizer.
The polymer used to form the PEM should fulfill a number of conditions relating to mechanical, physio-chemical, and electrical properties. First, the polymer should exhibit ion exchange properties that allow sufficient conductivities to be achieved between the anode and cathode. For example, the polymer should exhibit a conductivity of at least about 0.05 mS/cm at operating conditions. In addition, the polymer should exhibit high chemical, dimensional, and mechanical stability during preparation and under extreme operating conditions, which are typically encountered in many fuel cell applications. For example, the polymer used to form the PEM should allow essentially no permeation of the fuels used in the fuel cell through the PEM. Moreover, it is desirable that the polymer used to form the PEM should be essentially water insoluble and resistant to swelling.
The polymer most widely used as a PEM for the manufacturing a fuel cell is NAFION, which is commercially available from DuPont. Polymers of NAFION are typically obtained by the co-polymerization of two fluorinated monomers, one of which carries a sulfonic acid (SO3H) group after hydrolysis. NAFION is adequate for use in many current fuel cell applications, but exhibits several deficiencies. NAFION exhibits structural instability at temperatures above 100° C. Moreover, NAFION has poor conductivity at low relative humidity and can not readily be used at temperatures above 80° C. because it dries out. Furthermore, NAFION exhibits high osmotic drag, which contributes to difficulties in water management at high current densities. In addition, high methanol permeability in NAFION contributes to detrimental fuel cross over, in which fuel passes across the anode, through the NAFION membrane and to the cathode. Consequently, in instances of fuel cross over, methanol is oxidized at the cathode and fuel cell efficiency decreases.